1. Field of the Invention
The present invention relates to the field of organic semiconductor and organic photovoltaics.
2. Related Art
A major trend in the development of high performance organic semiconductors is reflected by the recent burst of research efforts on low bandgap small molecules and polymers. As a common structural feature of the majority of such materials, alternating electron-rich (donor) and electron-deficient (acceptor) units are linked along an electroactive backbone. Their highly modular synthesis protocols, which rely on potent metal catalyzed cross-coupling reactions between electron donors and acceptors, essentially provide access to an unlimited number of functional materials for applications in organic field effect transistors (OFETs) and organic photovoltaics (OPVs). Along the lines of materials discovery, the search for efficient electron donor and acceptor units is among the most critical steps for better control of key materials parameters, such as electronic energy levels, optical bandgaps and absorptivity, and thin film morphology.
FIG. 1a illustrates structures of several commonly used electron deficient building blocks. Over the years, several popular electron acceptors have become the workhorses in the development of high performance low bandgap materials, such as benzothiadiazole (BTD), diketopyrrolopyrrole (DPP), isoindigo (iI), benzobisthiadiazole (BBT), and those based on rylene diimides. Small molecules and polymers incorporating these units have displayed impressive power conversion efficiency in organic photovoltaics (OPVs) and remarkable charge transport mobilities in organic field effect transistors (OFETs).
Some of these acceptors, such as DPP and iI, are based on industrial pigments, which add extra value in terms of materials stability. The centrosymmetric ketopyrrole cores of DPP and iI are, however, not readily available from naturally occurring pigments and have to be built through a few synthetic steps. On the other hand indigo, as one of the oldest known dyes, possesses a similarly appealing symmetric ketopyrrole structure and is a low cost pigment material (several US dollars per kilogram) that can be obtained from natural sources or synthetically. Despite the fact that native indigo and its derivatives have received more attention for their high performance in ambipolar FET transistors, their direct use as electronic materials is restricted, largely due to their limited solubility. Functionalization of indigo would both increase its solubility and modify its electronic properties. FIG. 1b illustrates different motifs of functionalizing Indigo. The bonds in the hatched rectangle highlight the conjugation pathway in the BAI core.